Assessing sustainable groundwater abstraction: an evaluation of impacts on groundwater quantity and quality

Abstract

Groundwater is often a stable, clean and important drinking water resource, and in many places around the world, it is a prerequisite for economic growth; in fact, 50% of the world’s population depends on groundwater. However, bad abstraction management endangers availability and quality, and so in order to ensure future water security, there is a need for trustworthy, reliable and accurate assessment methods to identify the impacts on groundwater resources. Internationally, there are good indicators for evaluating general water stress on a large scale, but with ever-increasing pressure on companies to evaluate and communicate their environmental footprint, water utilities also need tools to evaluate groundwater stress on a local scale. This PhD project proposes new indicators for evaluation of local impacts on groundwater abstraction. Secondly, it explores the relationship between groundwater drawdown and water quality. Finally, it develops a new method for integrating considerations of groundwater quality into an assessment of sustainable groundwater abstraction. This thesis discusses current indicators, their challenges, and potential solutions. One of the indicators proposed, AGWaRe, which evaluates Available GroundWater Remaining for other users, is based on the principles of an existing and internationally accepted indicator, AWaRe. AGWaRe is developed so that it can evaluate groundwater and be applied on a local scale. AGWaRe and other indicators include environmental groundwater requirement (EGWRs), recognizing that water abstraction affects ecosystems. Generally, EGWRs and thereby existing indicators are based on evaluating quantitative changes to stream flows and aquifer recharge, and they do not take into consideration the effect on water quality caused by abstraction. To evaluate how groundwater abstraction affects groundwater quality, the correlation between drawdown and water quality was analysed based on data from 1900 – 2014 for 28 well fields supplying water to Copenhagen. It showed that for these well fields the development in sulphate concentrations can indicate overall sustainable groundwater abstraction. Changes in sulphate concentrations indicated that the water abstraction in the 1980s, when it was at its highest, was unsustainable, because sulphate concentrations were increasing steadily. The results highlight that groundwater abstraction has generally been sustainable at these well fields since the 1990s, because sulphate concentrations have been overall stable or decreased slightly. Recognizing that there is a limit to how much water can be abstracted from groundwater aquifers, if neither streamflow nor groundwater quality should be changed in an unacceptably degree, the following definition for EGWRs is proposed “water from groundwater resources needed to sustain flows, preserve groundwater dependent ecosystems and maintain good groundwater quality”. Hence, it is suggested to divide EGWRs in two categories, 1) EGWRflow, which is groundwater reserved to sustain base flow in streams and groundwater-dependent ecosystems and 2) EGWRwq, which is groundwater reserved to sustain water quality affected by water abstraction in the aquifers. The correlation between drawdown and changed water quality was used to model a safe groundwater abstraction for Zealand, Denmark. A conditioned drawdown was applied in the aquifers to secure stable groundwater quality. It showed that there is more groundwater available for abstraction, however the actual abstraction should be redistributed to secure a stable groundwater quality in all aquifers. Hence, it was possible to evaluate a groundwater abstraction for both groundwater quantity and quality. In addition, several suggestions are made in terms of how the utilities and other stakeholders can use and implement the findings in their search for sustainable groundwater abstraction.